This invention relates to a drum mixer asphalt plant used to produce a variety of asphalt compositions. More specifically, this invention relates to a drum mixer in which recycle material may be introduced at one or more feed locations and in which the region for the introduction of liquid asphalt and mineral fines is isolated from hot combustion gases used to dry and heat the aggregate material.
Several techniques and numerous equipment arrangements for the preparation of asphaltic cement, also referred by the trade as "hotmix" or "HMA", are known in the prior art. Particularly relevant to the present invention is the production of asphalt compositions in a drum mixer asphalt plant. Typically, water-laden virgin aggregates are heated and dried within a rotating, open-ended drum mixer through radiant, convective and conductive heat transfer from a stream of hot gases produced by a burner flame. As the aggregate material flows through the drum mixer, it is combined with liquid asphalt and mineral binder or "fines" to produce an asphalt composition.
Exposing the liquid asphalt to excessive temperatures within the drum mixer or in close proximity with the burner flame causes serious product degradation, in addition to health and safety hazards. As a result, various attempts have been proposed to help minimize combustion of the liquid asphalt necessary in the process.
Paddles or flighting mounted on the interior of the mixer have also been used to shield the liquid asphalt from the burner flame by creating a curtain of falling aggregate material disposed between the burner flame and the asphalt. While the flighting reduces the likelihood of combustion of the asphalt, the stream of hot gases emitted by the burner flame may still heat the asphalt to an excessive temperature. In such event, the more volatile components of the asphalt are released and the final product may become unfit for use in paving operations.
Excessive heating of asphalt compositions also results in a substantial air pollution control problem, known as "blue-smoke", caused when hydrocarbon constituents of asphalt are driven off and released into the atmosphere. Significant investments and efforts have been made by the industry in attempting to control blue-smoke emissions.
The use of cut-back asphalts containing diesel fuel in conventional drum mixers to produce cold-mix asphaltic cement also creates a considerable problem in that these asphalts are flammable, creating( the possibility of fires and potential explosions within the mixer when the asphalt is exposed to the burner flame or the excessive temperature of the gas stream.
Improvement is also needed in those drum mixers which recycle asphaltic cement removed from road surfaces. In these mixers, the recycle material is ground to a suitable size and mixed with the virgin aggregate prior to mixing with the asphalt. The presence of asphalt in the recycle material necessitates shielding the recycle material from the flame as well as the hot gas stream when the stream contains excessive temperatures.
The need remains in the asphalt industry for improved drum mixer design and operating techniques to address the problems and drawbacks heretofore experienced. The primary objective of this invention is to meet this need.